Various types of biometric systems are used more and more in order to provide for increased security and/or enhanced user convenience.
In particular, fingerprint sensing systems have been adopted in, for example, consumer electronic devices, thanks to their small form factor, high performance, and user acceptance.
Among the various available fingerprint sensing principles (such as capacitive, optical, thermal etc.), capacitive sensing is most commonly used, in particular in applications where size and power consumption are important issues. All capacitive fingerprint sensors provide a measure indicative of the capacitance between each of several sensing structures and a finger placed on or moved across the surface of the fingerprint sensor.
To accurately measure the capacitance between the finger and a sensing structure, it is desirable that the finger can be held at a known reference potential. In commonly available used in smartphones and the like, the reference potential can be provided by means of an electrically conductive bezel arranged around the fingerprint sensor, where a finger placed on the sensor also contacts the bezel.
However, for fingerprint sensor integration in smart cards, which is increasingly requested by the market, the requirements of the fingerprint sensor may change compared to when the sensor is used in a smartphone. For example, a bezel involves an individual manufacturing step and also adds steps to the assembly of the fingerprint sensor, thereby adding to the cost of the fingerprint sensor. Since the cost of a smartcard is significantly lower than the cost of smartphone, it is advantageous to also reduce the cost of the fingerprint sensor for a smart card. Furthermore, a rigid metal bezel cannot be incorporated in a flexible smart card.
Accordingly there is a need for an improved fingerprint sensor for integration in a smart card. In particular, there is a need for a fingerprint sensor where a controlled reference potential can be provided to a finger without the use of a bezel.